The CD4 receptor is a non-polymorphic glycoprotein having a molecular weight of about 60,000 daltons that is present primarily on the surface of T lymphocytes with helper/inducer function. Structurally, it consists of three segments: extracellular, transmembrane and cytoplasmic. The former comprises four immunoglobulin (Ig) variable (V) region-like domains of which the first, second and forth are linked by disulfide bonds. Because of sequence and structure homology with the V domain of Ig light (L) chain, it has been proposed that the gene coding for the CD4 molecule is a member of the Ig supergene family.
While it is clear that CD4 is a receptor that binds biologically important molecules, its physiological role is not fully understood.
Although it was long suspected that CD4 may function as a receptor molecule for MHC class II antigens, only recently was it demonstrated that fibroblasts transfected with a cDNA coding for CD4, and expressing high levels of this protein, bound tightly to human B cells bearing MHC-coded class II molecules. See for example, Guy et al., Nature, 328:626-629 (1987); Doyle et al., Nature, 330:258-259 (1987); and Sleckman et al., Nature, 328:351-353 (1987). This CD4-MHC Class II interaction was inhibited by either anti-CD4 or anti-MHC class II antibodies.
Several studies have provided evidence that the CD4 molecule acts as a receptor for the human immunodeficiency virus type 1 (HIV-1). For instance, Jameson et al., Science., 240:1335-1338 (1988) reported that the binding site for HIV-1 on CD4 is probably formed by amino acid residues 16-49.
Other receptors of interest to the present invention are those which bind the constant (Fc) portion of immunoglobulin molecules. For instance, bacteria of several gram-positive species produce proteins which bind to the Fc region of Ig. For example see the best known of these Fc receptors is protein A of Staphylococcus aureus. Protein A has been widely used in laboratory and clinical diagnostic immunochemical procedures which exploit the ability to bind to a variety of immunoglobulin G (IgG) antibodies independently of antigen association.
The gene coding for protein A has been cloned, its cDNA sequence determined and its amino acid residue sequence deduced. See LoFdahl et al., Proc. Natl. Acad. Sci. USA, 80:697-701 (1983); Uhlen et al., J. Biol. Chem., 259:1695-1702 (1984); and Moks et al., Eur. J. Biochem., 156:637-643 (1986). In addition, those portions of protein A responsible for IgG-binding have been identified by Moks et al., Eur. J. Biochem., 156:637-643 (1986), as consisting of five highly homologous segments, designated A-E, having sizes ranging from 50 (segment E) to 61 (segment D) amino acid residues.
Fc receptors with broader specificity than protein A are produced by Streptococcus species, especially those of Lancefield groups C and G. See for example, Bjorck et al., J. Immunol., 133:969-974 (1984); Langone, I.I., Adv. Immunol., 32:157-252 (1982); Myhre et al., Infect. Immunol., 17:475-482 (1977); and Reis et al., Mol. Immunol., 23:425-431 (1986). The protein produced by group G Streptococcus species known as protein G, has been shown to bind to all four classes of human IgG, including IgG3, to which protein A does not bind by Bjorck et al., J. Immunol., 133:969-974 (1984). Furthermore, protein G binds more strongly than protein A to several animal IgG classes and mouse monoclonal antibodies as demonstrated by Akerstron et al., J. Immunol., 135:2589-2592 (1985).
The gene for protein G has been cloned, its cDNA sequence determined and its amino acid residue sequence deduced. See Fahnestock et al., J. Bact., 167:870-880 (1986). Like protein A, the structure of the IgG-binding regions of protein G have been determined by Guss et al., EMBO J., 5:1567-1575 (1986). Those regions, designated C1, C2 and C3, each contain 55 amino acid residues and are separated within the protein by two "spacers", designated D1 and D2 of 16 amino acid residues each.